Hostas and Light

Hostas are often called “shade loving” plants. For the average gardening public, this helps Hosta-shade-lovingthem to understand that these plants are adapted to growing in the shade. However, for more advanced hobbyists or professionals, this is a misnomer.

All vascular plants live by a process called photosynthesis. They capture the energy from the sun and, unlike animals, they make their own food. For photosynthesis to work, a plant must have five things including light, water, nutrients from the soil and the air, chlorophyll and suitable temperature. The minimum levels of each of these factors required may vary from species to species but they all must be present in certain amounts for photosynthesis to take place in that plant.

For a plant to be truly a “shade lover” would imply that it could complete photosynthesis in a dark closet. That is clearly not the case here, so hostas are actually “shade tolerant” plants which can survive and often thrive at low (but not zero) light levels. In fact, most of them will do best in more sun but, to keep them aesthetically pleasing, we sometimes have to make concessions.

When people classify themselves as either a “shade” or a “sun” gardener, they often act as if we should all understand exactly what they mean. Here again, those terms can bring to mind a wide range of light conditions.  We will define garden light levels as follows:

_       1. Full Sun – Plants that are designated as full sun plants are usually those that are native to open, prairie or swampy type environments that support few or no trees. They are conditioned to having sunshine hit their leaves from sunup in the morning until sundown in the evening. That is the accurate definition of FULL SUN!

However, over the years, gardeners and horticulturists have come to understand that these plants can still perform adequately while being exposed to less than total sunlight all day long. The common rule of thumb is that you need a MINIMUM of around 6 hours of direct sunlight per day for these plants to thrive. Full sun plants that get less than that amount will develop stems that are overly long and weak i.e. etiolated, which tend to fall over. Such plants will not flower well and will not be as vigorous as those getting more sunlight. Just remember that those 6 hours are a minimum and more would be better.

_       2. Shade – So, if 6 hours is needed for full sun, it would be a simple conclusion to assume that shade plants can live and even thrive in less than 6 hours. Again, although we talk about shade as if it were a single unit, the reality is that there are generally considered to be 3 categories of shade in the home landscape.

_          a. Light ShaHostas_AAA_Shade-Lightde a.k.a. Dappled Shade or Partial Shade – This is the situation where you have just a few trees in the landscape or when you have trees with small leaves such as honeylocust. As the sun passes over the horizon, the plants in the shaded areas may receive a total of up to 5 or 6 hours of direct sunlight. The key is that they will get it a few hours here, an hour there and another hour later. It will not be 5 or 6 hours in a row. This will be a great growing environment for our shade tolerant plants.

_          b. Medium Shade a.k.a. Open Shade or High Shade – This is the situation when you have many large deciduous trees such as oaks where the bottom branchesHostas_AAA_Shade-Medium may be 20 or 30 feet off the ground. As the sun passes over head, the plants beneath may only receive direct sunlight for a couple of hours total during the day but the environment is generally “bright”. In this landscape, a person could take a seat under the trees and easily read a book but rarely have direct sunlight penetrate onto the paper. This is also a very good environment for shade tolerant plants.

Dense-shade_          c. Deep Shade a.k.a. Dark Shade or Dense Shade – Deep shade will occur beneath evergreen trees such as pines or spruce or deciduous trees with large leaves such as Norway maples. This will also be the case close to the north side of buildings. Under these conditions, as the sun moves across the horizon during the day, no direct rays will strike the plant. The only light will be that which is reflected back into the dark from the surroundings.

This is a very difficult environment for growing shade tolerant plants. Hostas and others will usually survive these conditions but they will not thrive. For instance, hostas growing under the first two levels (a & b) of shade will add new divisions to the clump at a rate consistent with that expected of a particular cultivar. Those same hostas grown in the deep shade will multiply at a much, much slower rate, if at all. They will also develop thinner and, often, fewer leaves than would the same cultivar planted in shade levels a and b.

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